What does "positive" mean regarding your blood type?

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The American Pregnancy Association explains that the presence or lack of the Rh factor determines whether a blood type is positive or negative. The Rh factor is an antigen and a protein type located on the surface of red blood cells that causes an immune system response. When individuals have the Rh factor, their blood is Rh-positive. When they do not have it, their blood is Rh-negative.

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According to the Canadian Blood Services website, the four major blood groups are A, O, B and AB. These four are then divided into Rh-positive and Rh-negative blood types. Determining a person's blood type is important because the compatibility of blood types varies. In Canada, O Rh-positive blood is the most common blood type, with the rarest being AB Rh-negative. The blood of people with O Rh-negative blood is suitable for transfusion to people with all other blood types. Children might have different blood types than their parents due to the combination of genes that occurs when parents conceive children, as each parent contributes one gene.

The University of Maryland Medical Center states that around 85 percent of Caucasians have Rh positive blood. Even higher percentages of African-Americans, Asians and American Indians have Rh positive blood.